OcrToBER 16, 1902] 
which the oyster was attached has usually disappeared, except 
in those cases in which it was attached to one of its own species. 
- More than half of the specimens have the two valves united and 
free from adhesions, so that they are capable of exhibiting the 
phenomenon referred to. More than nine-tenths of these show 
more or less clearly in the upper valve the figure impressed 
upon the lower valve by the shell to which the latter adhered. 
In most cases this is the figure of a part of the outer surface 
of a bivalve shell. In a few it is the inner surface of a bivalve 
shell. In one oyster the figure of part of a specimen of 
Cerithium magnicostatum, Conrad, is clearly shown above and 
- below ; in another, Cerithium libanoticum, Fraas, with a much 
better outer lip than is usually found in specimens of the original 
shell. In two cases the internal cast of a small Cerithium, 
together with some of the matrix, still adheres to the lower valve 
of the oyster, while its external form, lost below, is beautifully 
reproduced upon the upper valve. 
In two very striking instances, the lower valve of the oyster 
shows the impression of a bivalve shell with spiny ribs, while a 
reproduction of these same spiny ribs appears in high relief upon 
the upper valve. These reproductions in the upper valve of 
figures impressed upon the lower valve might be supposed to 
result from the close contact of two valves when both valves 
were thin and small, and might be expected to be confined to 
the region of the umbo in well-grown specimens; but in all 
cases in which the oyster has been attached by a large portion 
or all of the lower valve, the impression is reproduced upon a 
correspondingly large portion of the upper valve. In view of 
the fact that in most specimens the shell is from 1°5 to 2 cm. 
thick, and, further, that internal surfaces when exposed show no 
traces of these external markings, it is noteworthy that the 
markings should extend over so much of the upper surface 
instead of being confined to the umbonal region. 
ALFRED ELy Day. 
Syrian Protestant College, Beirut, Syria, September 22. 
THE peculiar phenomenon referred to in the above letter is 
well known to occur among Secondary fossils, and has been 
fully explained by Prof. J. W. Judd in the Geological Magazine 
for 1871, p. 385, where several figures of Oolitic forms are given 
in illustration, The same peculiarity is also seen in certain 
oysters from the Lias. The thin growing edge of the shell 
adapts itself to the inequalities of the surface upon which it 
grows; the upper valve, being also thin, reproduces the form of 
the lower valve. The shell becomes thickened by additional 
layers on the inside, which thus gradually loses the markings 
that are retained upon the outer surfaces. Bra aN. 
Refractivities of the Inert Gases. 
A RELATION appears to exist between the refractivities 
(u-1) of the inert gases of the atmosphere and that of hydrogen, 
which, so far as I am aware, has hitherto escaped attention. 
The following figures show that, taking the refractivity of 
hydrogen as 1, the refractivilies of the other gases are very 
nearly in the proportion of 4, 4, 2, 3 and 5. 
By far the largest divergence is in the case of helium. This 
gas, as I am informed, is difficult to purify from the admixture 
of the heavier gases, so that a perfectly pure specimen would 
probably give a better result. Evenif the relation to hydrogen 
is fortuitous, the ratios of the refractivities of the other five gases 
to one another are sufficiently interesting. 
Refractivities ! observed Ratio oe | Error 
(Air=1). | to H. enous | per cent 
H=0'4733- | 
2 | pee 
| | 
Helium [bod eke 0°1183 Sit 
INegnilieecsrae-t ssc s 0°2366 +0°9 
Hydrogen I | 
Argon tit fhe 0°9466 —2°2 
Krypton 3 1°420 —2°0 
Xenon 5 2°367 +o'1 
NATURE 
607 
Trade Statistics. 
In his reply to my letter (October 2, p. 550), Dr. Mollwo 
Perkin brings forward fresh figures, apparently proving an 
enormous decline in British industry since 1870-74. This, how- 
ever, is but to repeat Mr. Levinstein’s mistake in an aggravated 
form. The Franco-Prussian war in 1870 checked manufacturing 
abroad for a twelvemonth, and in 1870-74 there was a heavy 
demand for British iron and coal at excessively high prices. 
That period, as is well known, is useless for comparisons of 
British and German export trade. 
It is true,as Dr. Perkin points out, that the general rate of 
increase of exports (z.¢. of their total values irrespective of the 
number of producers) has, in the last twenty years, been very 
slow in this country, rather rapid in Germany and very rapid in 
the United States. 
But if we reckon per head of population, we get the following 
(from the Board of Trade *‘ Memorandum,” ‘‘ Cd. 1199”) :— 
Annual Exports (“ Special”) per head of Population. 
] 
Ave i | nited 
haigt i | ene Seve, || (So tuey: Sees 
| & fe cca Sell file 6 &£ 
1875-79 | Goon “ss, sins 2°81 
1880-84 6°66 3°67-% || 3°43 3°30 
1885-89 618 5-4 Ore |i 2°59 
1890-94. | 6°15 ae, 3257) 3°14 2°95 
1895-99 | S97 Ale SiS 3°36 | 292 
These figures are distinctly reassuring. They must not be 
used as an excuse for laxity in education or the application of 
science to manufacture, but they ought to allay unreasonable 
pessimism. 
The slight decline per head in the British exports (as 
measured in money, not in commodities) would be a rather 
unsatisfactory feature if the export trade were our chief trade 
and chief source of income. Dr. Perkin perhaps thinks that it 
is, for he translates Mr. Levinstein’s ‘* foreign trade” into 
“trade.” But the gross value of the export trade (280 or 300 
millions per annum), large as it is, is small compared with the 
total national income, recently estimated by Sir Robert Giffen 
at 1500 millions sterling, while the income-tax assessments 
indicate that it is increasing faster than the population (7Zz/es, 
May 23, 1901). This enormous income is, of course, chiefly 
made up of the value of goods produced and consumed within 
the country, constituting the internal trade as distinguished from 
the foreign trade. From this point of view a close scrutiny of 
export statistics appears to be unnecessary, and may easily be 
misleading. 
Unfortunately, we have no adequate statistics of total produc- 
tion. The figures for pig iron which Dr. Perkin gives are to 
the point, and the progress of our two great rivals is here very 
striking. But the pig-iron manufacture accounts for less than 
3 per cent. of our national earnings. F. EVERSHED. 
Kenley, Surrey, October ro. 
Material for Natural Selection. 
Verbesina exauriculata’ is an evil-scented but handsome 
herbaceous plant with broad orange rays, very abundant in the 
town of Las Vegas, New Mexico. My class in biology has been 
making a study of the variations in the number of rays in the 
heads of this plant, and in so doing we took occasion to compare 
two sets, from the eastern and western parts of the town 
respectively. The result was as follows, calling these sets Aand 
B respectively :-— 
we }o (8) (9) (to) (xx) (x2) (13) (14) (15) (16) (27) (18) (19) (20) (2 
Number of 
heads, set A 
Number of 
heads, set B. 
= ae Tie SOR OOLESONNTO tau 7 erie ny) me neEae 
intr (oral zg iigzni73) 84hsuixzh sg2 he ee 
CLIVE CUTHBERTSON. 
9 York Terrace, Regent’s Park, N.W., October ro. 
1 Ramsay and Travers, Pil. Trans., vol. excvii. A, 1901, p. 47. 
NO. 1720, VOL. 66] 
1 Verbesina encelioides exauriculata, Robinson and Greenman, Prec. 
Amer. Acad., May, 1399, p. 544- Notwithstanding the name of this 
northern type, the petioles of the upper leaves are commonly strongly 
auriculate. 
